Our previous research and that of others show that present models of visual processing are not satisfactory in suprathreshold vision, when a background is present. The proposed research seeks to advance our understanding of suprathreshold vision in known and unknown (noise) backgrounds. We seek to understand contrast processing, visual plasticity and long-range interactions using physiologically plausible mechanisms in normal and amblyopic subjects. Aim 1 develops and refines new methods for experimentally partitioning several factors of noise masking. A highly efficient procedure is introduced for measuring templates and noise in early stages of pattern processing. Aim 2 makes use of these methods for testing a number of hypotheses regarding the detection of a broad class of stimuli in noise. These results have relevance to our understanding of human spatial vision and to many real world tasks such as detecting tumors in medical images and image compression algorithms. Aim 3 is devoted to answering fundamental questions about the factors that limit our ability to perceive changes in contrast. Two types of perturbation will be used to isolate the underlying mechanisms of contrast processing: adaptation and long-range influences of surrounding stimuli. Special attention will be paid to the connection between the perceived contrast and the ability to distinguish contrasts of a test pattern. These studies will be specifically directed to making connections with findings from neurophysiology. Aim 4 applies the basic research findings from the previous aims to perceptual learning. Over the past decade evidence has been gathering that nervous system plasticity enables learning of pattern detection and discrimination with a specificity that indicates early or pre-decision stage learning. The proposed experiments carefully test the hypothesis that the learning is early rather than at the decision stage. Aim 5 applies our methods to gaining a deeper understanding of the amblyopic deficit. Of particular interest is the nature of the perceptual distortion at suprathreshold vision as a manifestation of early cortical organization. The understanding of spatial vision has progressed rapidly such that we can now accurately predict detection thresholds for arbitrary patterns on blank backgrounds using standard filter models. The proposed research will extend our understanding of spatial vision to suprathreshold levels where current models have limited utility. The focus is on understanding contrast processing, visual plasticity and long-range interactions in normal and amblyopic subjects, within a guiding framework of physiologically plausible mechanisms.